The suspension polymerization of vinyl chloride is generally carried out at temperatures below 70.degree. C. using organic soluble initiators. Although lauroyl peroxide was earlier the most widely used catalyst, in recent years other low temperature catalysts including azobisisobutyronitrile, diisopropyl peroxydicarbonate, t-butyl peroxypivalate and mixtures thereof, have been adopted. These and other catalysts are described in Pennwalt Corporation, Lucidol Division, Technical Bulletin 30.90, "Free Radical Initiators for the Suspension Polymerization of Vinyl Chloride" (1977).
The choice of initiator is dictated by its half-life and by its influence on the polymerization process and the properties of the poly(vinyl chloride) produced thereby.
The polymerization of vinyl chloride is characterized by a short induction period, followed by a gradually increasing rate of polymerization. During the earlier stages of the polymerization, the reaction rate is lower than the maximum so that the capacity of the reactor is not fully utilized. Peroxyesters reduce the induction period and, due to a more constant rate of polymerization, increase reactor productivity. Further, peroxyesters can generally be used at levels below that needed for peroxides and give much less chain branching during polymerization.
Although peroxyesters such as diisopropyl peroxydicarbonate and t-butyl peroxypivalate offer numerous advantages in vinyl chloride polymerization, their disadvantages include the necessity for low temperature shipping and storage and decreased efficiency at elevated temperatures.
The use of peroxyesters having higher decomposition temperatures is not feasible in present poly(vinyl chloride) production facilities due to the higher monomer pressures involved and the low molecular weight and poorer stability of the resultant resins. Nevertheless, the handling advantages of such peroxyesters makes their use extremely attractive.
The use of higher temperature catalysts at lower temperatures is a common practice in polymer technology. Thus, redox systems such as ammonium persulfate-sodium metabisulfite and hydrogen peroxide-ferrous sulfate are used in emulsion polymerization while benzoyl peroxide-dimethylaniline and methyl ethyl ketone peroxide-cobalt naphthenate are used in styrene-unsaturated polyester polymerization.
Reducing agents used in conjunction with monomer-soluble peroxyesters in the polymerization of vinyl chloride include potassium metabisulfite (N. Fischer and C. Lambling, French Pat. No. 2,086,635 (1972), sodium bisulfite (H. Minato, K. Hashimoto, and T. Yasui, Japan. Pat. No. 68 20,300 (1968), sodium bisulfite-cupric chloride (B. K. Shen, U.S. Pat. No. 3,668,194 (1972), sodium dithionite-ferrous sulfate (H. Minato, Japan. Pat. No. 70 04,994 (1970) and trialkyl boron (R. Kato and I. Soematsu, Japan. Pat. No. 5498('65) (1965); A. V. Ryabov, V. A. Dodonov, and Y. A. Ivanova, Tr. Khim. Khim. Tekknol., 1970, 238; Stockholms Superfosfat Fabriks A/B, Brit. Pat. No. 961,254 (1964).
Reducing agents used in conjunction with monomer-soluble diacyl peroxides in the polymerization of vinyl chloride include ferrous sulfate-sodium hydroxide (A. M. Sharetskii, S. V. Svetozarskii, E. N. Zil'berman, and I. B. Kotlyar, Brit. Pat. No. 1,164,250 (1969) and U.S. Pat. No. 3,594,359 (1971), ferrous caproate (J. Ulbricht and N. V. Thanh, Plaste Kaut., 21, 186 (1974); J. Ulbricht and G. Mueller, Plaste Kaut., 21, 410 (1974) and trialkyl boron (A. V. Ryabov, V. A. Dodonov, and Y. A. Ivanova, Tr. Khim. Khim. Tekknol., 1970, 238).
Various stannous compounds have been disclosed as reducing agents in the suspension polymerization of vinyl chloride in the presence of peroxyesters and diacyl peroxides in our copending applications. Thus, stannous carboxylates are disclosed in application Ser. No. 106,332, filed Dec. 21, 1979, now U.S. Pat. No. 4,269,957; stannous carboxylate-complexing agent combinations are disclosed in application Ser. No. 106,325, filed Dec. 21, 1979, now U.S. Pat. No. 4,269,956; stannous mercaptides are disclosed in application Ser. No. 106,331, now U.S. Pat. No. 4,242,482 (1980); stannous chloride has been disclosed in application Ser. No. 106,333, filed Dec. 21, 1979, now U.S. Pat. No. 4,269,958.
Monosaccharides and monosaccharide carboxylic acid esters have been disclosed as reducing agents in the suspension polymerization of vinyl chloride in the presence of peroxyesters in our U.S. Pat. No. 4,261,870 (1981).
Ascorbic acid has been used as the sole reducing agent or in combination with cupric, ferrous or ferric salts, in the polymerization of vinyl chloride in the presence of water-soluble catalysts including hydrogen peroxide (H. I. Roll, J. Wergau and W. Dockhorn, Ger. Offen. Pat. No. 2,208,442 (1973); J. A. Cornell, U.S. Pat. No. 3,534,010 (1970); K. Okamura, K. Suzuki, Y. Nojima and H. Tanaka, Japan. Pat. No. 18,954('64) (1964); H. Watanabe, S. Yamanaka and Y. Amagi, Japan. Pat. No. 16,591('60) (1960), potassium persulfate (K. H. Prell, E. Plaschil and H. Germanus, East Ger. Pat. No. 75,395 (1970), cumene hydroperoxide (R. J. S. Matthews, Brit. Pat. No. 931,628 (1963), acetyl cyclohexanesulfonyl peroxide (Dynamit Nobel A.G., Netherlands Appl. No. 6,408,790 (1965), and a mixture of hydrogen peroxide and acetyl cyclohexanesulfonyl peroxide (R. Buning, K. H. Diessel and G. Bier, Brit. Pat. No. 1,180,363 (1970).
Ascorbic acid has been disclosed as a complexing agent in the polymerization of vinyl chloride in the presence of a diacyl peroxide and various water-soluble metal salts (N. Fischer, J. Boissel, T. Kemp and H. Eyer, U.S. Pat. No. 4,091,197 (1978). The complexing agent "causes the metal to change from its water-soluble form to an organo-soluble form" so that it can function as an "activator" for the organo-soluble initiator. There is no teaching that water-soluble ascorbic acid is capable of acting as a reducing agent for the water-insoluble initiator in the absence of the metal salts.
Isoascorbic acid, also known as erythrobic acid, D-ascorbic acid and D-araboascorbic acid, as well as ascorbic acid are disclosed as reducing agents in combination with the salts of heavy metals such as iron, copper, tin, silver and cerium, in the polymerization of vinyl chloride in alcohol solution, in the presence of various peroxygen compounds (Kureha Chemical Industry Co., Ltd., Japan. Pat. No. 7493('60) (1960). Similarly, isoascorbic acid and ascorbic acid are jointly disclosed as components of catalyst systems containing peroxides and either heavy metal salts or metal chelates, for the emulsion polymerization of vinyl bromide in water or aqueous alcohol (I. Shinohara and J. Aoyagi, Japan. Kokai No. 73 102,887 (1973); E. Tsuchida, Japan. Kokai No. 73 20,884 (1973).
The use of an alcoholic medium permits the homogeneous polymerization of vinyl acetate in the presence of t-butyl perbenzoate with isoascorbic acid or ascorbic acid as part of a redox catalyst system (Japan Synthetic Chemical Industry Co., Ltd., Japan. Pat. No. 10,593('62) (1962).
The prior art cited hereinabove discloses the use of water-soluble ascorbic acid as either the sole reducing agent or in combination with water-soluble metal compounds, in the emulsion polymerization of vinyl chloride, generally in the presence of a water-soluble peroxygen compound. It further discloses that either water-soluble ascorbic acid or water-soluble isoascorbic acid may be used in the polymerization of vinyl chloride, vinyl bromide or vinyl acetate in the presence of a peroxygen compound in water or aqueous alcohol. There is no disclosure of the use of ascorbic acid or, more particularly, isoascorbic acid, as the sole reducing agent, in the absence of alcohol and/or metal salts, in the suspension polymerization of vinyl chloride in the presence of water-insoluble peroxygen compounds.
6-O-Palmitoyl-L-ascorbic acid has been used as reducing agent in the polymerization of vinyl chloride in the presence of hydrogen peroxide (K. Kamio, T. Tadasa and K. Nakanishi, Japan. Pat. No. 71 07,261 (1971) and methyl ethyl ketone peroxide (K. Kamio, T. Tadasa and K. Nakanishi, Japan. Pat. No. 70 25,513 (1970).
The bulk or suspension polymerization of vinyl monomers, particularly vinyl chloride, in the presence of a peroxyester or a diacyl peroxide and a 6-O-alkanoyl-L-ascorbic acid has been disclosed in our copending application Ser. No. 106,330, filed Dec. 21, 1979, now U.S. Pat. No. 4,269,960. However, this redox catalyst system requires relatively high concentrations of the peroxygen compound to obtain commercially acceptable polymerization rates, and consequently, the poly(vinyl chloride) contains residual peroxygen compound which is detrimental to the thermal stability of the polymer.